The invention provides platinum or platinum alloy powders, preferably platinum/ruthenium alloy powders and a process for preparing these. The metal powders contain very little chlorine, have a high surface area and are preferably used as catalysts in chemical reactions (such as for example hydrogenation) and in fuel cells.
It is known in the art that platinum/ruthenium catalysts are useful as anode catalysts for sulfuric acid fuel cells. These catalysts have a low tendency to be poisoned by carbon monoxide. They have also proven useful for the oxidation of methanol and are thus suitable for direct methanol fuel cells (DMFC=Direct Methanol Fuel Cell). They are frequently prepared by the so-called Adams-Shriner process.
Adams and Shriner describe the preparation of platinum-(IV)-oxide, (J. Am. Chem. Soc. 45,2171 (1923)). The preparation and catalytic effect of binary oxides, for example Pt/Ru oxide, are described by P. N. Rylander et al. (Engelhard Industries, Tech.Bull 8, 93 (1967)). The oxides prepared by this process can be reduced to the corresponding noble metal blacks by reducing agents such as hydrogen, formaldehyde or hydrazine. A metal black is understood to be a finely divided metal powder with a high specific surface area.
According to the Adams-Shriner process, a salt mixture which consists of hexachloroplatinic-(IV)-acid and an excess of sodium nitrate is melted in a quartz crucible and heated to 500xc2x0 C. The melt is maintained at this temperature for a defined time and then cooled down. The solidified melt is dissolved in water and a residue of platinum oxide remains. This is filtered off and washed. The residue is then suspended and reduced to platinum black by reduction with a suitable reducing agent, for example hydrazine.
By using different chlorides of the platinum group metals (PGM) such as, for example, hexachloroplatinic-(IV)-acid and ruthenium-(III)-chloride, a mixture of the PGM oxides can be prepared in the desired ratio and then reduced to the metals.
The Adams-Shriner method of preparation has a number of inherent processing disadvantages: The process is based on first reacting the chlorine-containing noble metal compounds with sodium nitrate to give the corresponding noble metal nitrates which are then decomposed at elevated temperature to give the noble metal oxides. Large amounts of nitrogen oxides are produced during the decomposition process. Thus, the process pollutes the environment and is hazardous to one""s health. Due to the intense generation of gas during the melting process, the melt tends to foam and has to be kept under constant control. Hexachloroplatinic-(IV)-acid which has not yet reacted becomes entrained in the escaping gas. This requires the use of considerable safety precautions during the entire process due to the allergenic effect of hexachloroplatinic-(IV)-acid.
The batch size can be increased to only a limited extent with this method. Adams and Shriner describe trials with mixtures of 4.2 g of hexachloroplatinic-(IV)-acid and 40 g of sodium nitrate which can be heated to the desired final temperature of, for example, 500xc2x0 C. within a few minutes. With larger batches, substantially more time is required in order to achieve a homogeneous melt. In this case, some regions of the salt mixture have already been at the set-point temperature for a long time while other regions have still not melted. Since both the temperature and the residence time at that temperature have an effect on the particle size and the surface area of the oxides formed, considerable growth in the oxide particles has sometimes already taken place. Therefore, after reduction of these oxides to the metals, inhomogeneous, coarse-grained powders with a wide distribution of particle sizes and correspondingly low surface areas are obtained.
After solidification of the melt, it has to be knocked or dissolved out of the crucible, which requires considerable effort.
Due to the use of chlorine-containing starting materials, the end product still has a considerable concentration of chlorine, which is undesirable for industrial applications, in particular as fuel cell catalysts. In this case, high chlorine contents lead to corrosion problems and shorten the working life of fuel cell systems.
Based on the forgoing, there is a need in the art for a process for preparing platinum or platinum alloy powders which produces metal powders with uniform, finely divided particle sizes, high surface areas and low chlorine contents, even with large batches. In addition, the process is intended to be more environmentally friendly by reducing nitric oxide emissions and to minimize the risk of triggering allergies with regard to chlorine-containing platinum compounds. There is also a need in the art for platinum or platinum alloy powders with low chlorine content prepared by this process.
The present invention provides a platinum or platinum alloy powders and a process for preparing these. Accordingly, the present invention provides a platinum or platinum alloy powder for use as a catalyst in fuel cells or in chemical reactions, comprising a powder that has a BET specific surface area greater than 40 m2/g and has a chlorine content of less than 100 ppm.
For a better understanding of the present invention together with other and further advantages and embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.